Modulation system



March 3, 1.942. NELSON 2,275,065

MODULATION SYSTEM Filed Oct. 19, 1940 FIG! MODULATION INPUT 8+ MODULATED OUTPUT CARRlER-SiGNAL INPUT 9 0 ll l2 f L (22 l3 4 IMPEDANGES J OF MODULATOR TUBES g; Z| o- INPUT IMPEDANCE 0F \2/ EITHER AMPLIFIER TUBE 9L1 F1e.2

UNMODULATEQ CARRIER-SIGNAL VOLTAGE INPUT Patented Mar. 3, 194:

2,275,065 MODULATION SYSTEM Arthur L. Nelson, Fort Wayne, Ind., assignor to Farnsworth Television and Radio Corporation, a corporation of Delaware Application October 19, 1940, Serial No. 381,815

11 Claims.

This invention relates to signal-modulating systems and more particularly to a modulatin system comprising a balanced modulator.

Modulating systems of a preferred type generally comprise'a modulated stage including one or more carrier-signal repeating or amplifier tubes connected in circuit with one or more modulator tubes. A modulating signal is applied to the modulator tubes, changes their operating conditions accordingly, and hence also changes the operating conditions of the carrier-signal repeating or amplifier tubes, whereby modulation of the carrier signal is effected in accordance with the modulating signal. a

In such arrangements, the modulated stage preferably comprises two amplifier tubes connected in push-pull relation to the control grids of which carrier-signal voltages of equal magnitude but opposite phase are applied. The potentials of these control grids are also varied in accordance with a modulating signal applied to the modulator stage. Since the latter grid potential variations must be in the same phase, an unbalanced'modulator is generally employed. It would, however, be desirable to use a balanced push-pull circuit connection in the modulator stage to take advantage of the greater undistorted degree of modulation readily obtainable taken in connection with the accompanying drawing, and its scope will be pointed out in th appended claims.

In the accompanying drawing:

Fig. 1 is a circuit diagram of a modulating sys tem embodying the present invention; and

Fig. 2 shows curves illustrative of the operation of the system.

Referring now more particularly to Fig. 1, there is provided a modulated signal repeater or amplifier comprising a pair of electron discharge tubes I and 2. The tube I has a cathode 3, a control grid 4 and an anode 5, while the tube 2 includes a cathode 6, a control grid I and an anode 8. For the purpose of applying carrier-signal voltages of opposite phase to the control grids 4 and I, there is connected thereto a pair of input terminals 9 and III adapted to receive carrier signals. Terminals 9 and I0 are also connected to the open ends II and I2 of a transmission line I3, which has an electrical length equal to onequarter of the carrier-signal wave length. The closed end I4 01 the line I3 is connected to ground by way of a grid-leak resistor I5 common with such a circuit, as well as the reduction of the effective input capacitance 01' such a connection.

It is an object of the present invention, therefore, to provide a new and'improved method of and means for modulating carrier signals, including the use of a balanced modulator stage.

In accordance with the present invention, there is provided a signal-modulating system comprising a signal-repeating means having an input circuit to which a carrier signal is applied to control the signal-repeating means in accordance therewith. There is also provided a modulator stage comprising a pair of variable impedances and means for varying these impedances in opposite phase in accordance with a modulating signal. A coupling means including an impedance-inverting member is provided for coupling the variable impedances to the input circuit of the signal-repeating means for the purpose of controlling the latter in accordance with the modulating signal. Finally, there is provided an output circuit in the signal-repeating means in which a modulated carrier signal is developed.

For a better understanding of the invention, together with other and'iurther objects thereof, reference is made to the following description to the circuits including the control grids 4 and 1. Connected to the tubes I and 2 is an output circuit comprising a transmission line I6 having its open ends I! and I8 connected to the anodes 5 and 8 and its closed end connected at I9 to a source of anode-voltage supply indicated at-B+.

For the purpose of modulating the carrier signal in accordance with a modulating signal, there is provided a pair of modulator tubes 20 and 2i connected in push-pull relation as shown. The tube 20 is provided with a cathode 22, a control grid 23 and an anode 24, while the tube 2i includes a cathode 25, a control grid 26 and an anode 21. A pair of input terminals 28 and 29 is provided for receiving a modulating signal. The terminals 28 and 29 are coupled to the control grids 23 and 26 of the modulator tubes 20 and 2| by way of the coupling condensers 30 and SI and grid-leak resistors 82 and 33, respectively.

control grid 4 of-the tube I is connected to the anode 21 of the tube 2| by way of a condenser 36 and an unbalanced transmission line 31 having an electrical length equal to one- 'quarter of the carrier-signal wave length, while the control grid I of the tube 2 is connected to the anode 24 of the tube 2!! by way of a condenser 38 and an unbalanced transmission line 39 having an electrical length equal to one-half of the carriersignal wave length. It will be noted that an impedance connected to one end of the line 31 is reflected at the other end with its inverted value, while this is not the case with the line 39.

Conventional means may be employed for heating the cathodes 3, S, 22 and 25, these means not being shown in the drawing to avoid unnecessary complication thereof.

Referring now to the operation of the system, a carrier-signal voltage, as indicated by the curve V1 of Fig. 2, is applied to the input terminals 9 and III andhence to the control grids 41 and l of the tubes I and 2, respectively, thereby to control the tubes I and 2 in accordance with the carrier signal and to develop a carrier-output signal in the transmission line I6 connected to the anodes and 8 of the tubes 8 and 2, respectively.

A modulating signal is applied to the terminals 28 and 29 and thereby to the control grids 23 and 2B of the modulator tubes and II. The internal output impedances Z1 and Z2, respectively, of these tubes then vary in accordance with the modulating signal. Since the modulator tubes 20 and 2| are connected in push-pull relation, their internal output impedances Z1 and Z2 vary by an equal amount but in opposite phase when a modulating signal is applied, as shown by the curves Z1 and Z2 of Fig. 2 for the case of a.

sinusoidal modulating signal. Viewed from the amplifiertubes I and 2, the output impedances of the modulator tubes 20 and 2| may be considered to be coupled in parallel relation with respect to the input impedances of the tubes I and 2. While the impedances Z1 and Z2 of the modulator tubes 20 and 2| are varied in opposite phase in accordance with a modulating signal, the variations in the input impedances Z of the amplifier tubes I and 2 will be in the same phase, due to the impedance-inverting characteristic of the quarter-wave length transmission line 31, as shown by the curve Z of Fig. 2. The variations of the input impedances Z of the amplifier tubes I and 2 cause a variation in the magnitude of the carrier-signal voltage applied to the control grids 4 and I of the amplifier tubes I and 2. Hence, when the input impedance Z of either of the amplifier tubes I and 2 decreases, the carrier-signal input voltage also decreases, and vice versa. In this manner, a carrier-signal output voltage is developed in the output circuit comprising the transmission line I6, which is modulated in accordance with a modulating signal applied to the terminals 28 and 29 of the modulator tubes. This modulated output-signal voltage is indicated by the curve V2 of Fig. 2.

In the present embodiment of the invention,

shown as an impedance-inverting element. Obviously, any other element having the same characteristics is deemed to be a full equivalent thereof. Likewise, multiple-purpose tubes in which more than one electron'discharge system is enclosed within a single envelope can be used instead of a plurality of separate tubes in the modulator stage and the modulated stage.

It will be understood that, at the lower frequencies, ordinary resonant circuits employing lumped reactances may be substituted for the transmission lines in the input and output ciraevaoee nal-repeating means having an input circuit,

means for applying a carrier signal to said input circuit to control said signal-repeating means in accordance therewith, a pair of variable impedances, means for varying said impedances in opposite phase in accordance with a modulating signal, plural means including one impedanceinverting member for coupling said impedances to said input circuit to control the operation of a quarter-wave length transmission line has been i means with inversion of the impedance thereof to said input terminal means, and an output circuit connected to said output terminal means for developing therein a modulated carrier signal.

3. A signal modulating system comprising a 7 pair of signal-repeating means each having input and output terminals, means for applying a carrier signal to the input terminals of said signal-repeatingmeans, means for connecting one of said varying impedance means without inversion of the impedance thereof to the input terminals of one of said signal-repeating means, means for connecting the other of said varying impedance means with inversion of the imped-' ance thereof to the input terminals of the other of said signal-repeating means. and an output circuit connected to said output terminals for developing therein a modulated carrier signal.

4. A signal modulating system comprising sigrial-repeating means having input and output terminal means, means for applying a carrier signal to said input terminal means, a pair of'impedance means adapted to be varied in opposite senses in accordance with a modulating signal, means for connecting one of said varying impedance means without inversion of the impedance thereof to said input terminal means, a transmission line for connecting theother of said varying impedance means with inversion of the impedance thereof to said input terminal means, and

w an output circuit connected to said output tersignal, means for connecting one of said varying impedance means without inversion of the impedance thereof to said input terminal means, a transmission line having an effective length equal to one-quarter the wave length of said carrier signal for connecting the other of said varying impedance means with inversion of the impedance thereof to said input terminal means, and an output circuit connected to said output terminal means for developing therein a modulated-carrier signal.

6. -A signal modulating system comprising signal-repeating means having input and output terminal means,v means for applying a carrier signal to said input terminal means, a pair of impedance means each including a vacuum tube adapted to be varied in opposite phase in -accordance with a modulating signal, means for connecting one of said varying impedance means without inversion of the impedance thereof to said input terminal means, means for connecting the other of said varying impedance means with inversion of the impedance thereof to said input terminal means, and an output circuit connected to said output terminal means for developing therein a modulated carrier signal.

7. A signal modulating system comprising a first pair of signal-repeating vacuum tubes each having an input electrode and an output electrode, a second pair of signal-repeating vacuum tubes each having an input electrode and an output electrode, means connecting one of said second-output electrodes to one of said first input electrodes, impedance-inverting means connecting the other of said second output elecapplying a modulating signal to said second input electrodes, and an output circuit connected to said first output electrodes for developing theretrol grid and an anode, a second pair of vacuum tubes each having a control grid and an anode,

means for applying a carrier signal to said first control grids, means for applying a modulating signal to said second control grids, a transmission linehaving an effective length equal to one-half the wave length of said carrier signal connecting one of said second anodes to one of said first control grids, a transmission line having an effective length equal to one-quarter the wave length of said carrier signal connecting the other of said second anodes to the other of said first control grids, and an output circuit connected to said first anodes for developing therein a modulated carrier signal.

10. A signal modulating system comprising a pair of amplifier vacuum tubes each having a control grid and an anode, a pair of modulator vacuum tubes each having a control grid and an anode, means for applying a modulating signal to said modulator control grids, means for trodes to the other of said first input electrodes, means for applying a carrier signal to said first input electrodes, means for applying a modulating signal to said second input electrodes, and

an output circuit connected to said first output.

transmission line, means for applying a carrier signal to said first input electrodes, means for applying a carrier signal to said amplifier control grids, a transmission line having an effective length equal to one-half the wave length of said carrier signal connecting one or said modulator anodes to one of said amplifier control grids, a transmission line having an e'fiective length equal to one-quarter the wave length of said carrier signal connecting the other of said modulator anodes to the other of said amplifier control grids, and an output circuit connected to said amplifier anodes for developing therein a modulated carrier signal, said output circuit comprising a transmission line having an eflective length equal to one-quarter the wave length of said carrier signal.

11. The method of producing modulated signals which comprises applying a carrier signal to a signal-repeating means, varying a pair of impedances in opposite phase in accordance with a modulating signal, coupling one of said varying impedances without inversion thereof to said signal-repeating means, coupling the other of said varying impedances with inversion thereof to said signal-repeating means, and utilizing the resultant modulated output signal of said signalrepeating means;

' ARTHUR L. NELSON. 

